How can I make a hole in this mesh and not mess it up?
This looks like a good candidate for Bezier curve objects.

It looks like you modeled you "blade" as a mesh object, there is nothing inherently wrong with it, but what you are trying to achieve may be a lot eaiser to do with Bezier Curve objects

They describe mathematical curves controlled by handles, instead of vertex, edges, and faces like meshes.

Since you don't have to actively worry about topology or "filling" in the faces they are very easy to use for opening holes like you want.



They will however require that you start to model from scratch using a diferent object.

Have in mind that they may not be useful depending on what you intend to use your object for later. If you plan on doing heavy texturing work involving unwrapping your mesh, shape animations, rigging, or plan to export this into a game engine; the topology produced by Bezier Curve Objects may not be adequate.

Otherwise you may also do this using meshes. Whether using Booleans (not recommended) or manually modelling it, but it will require lots of work and good understanding of proper topology.